COMP309/509 - Parallel and Distributed Computing

Lecture 2 - Processes in Unix

By Mitchell Welch

University of New England

Reading

Chapter 1, 7 and start chapter 8 from Advanced Programming in the UNIX environment
UNIX Programming Tools - should be mostly revision
Assignment 1 Description
COMP309/509 Study Guide - Pay Special Attention to the Assessment and Study Plan

Summary

The Structure of a Program
Process Control Basics
The fork() System Call
Examples

The Structure of a Program

Every C program has a main function:

int main( int argc, char* argv[]);

This function is specified as the staring address for the program in the linking process during compilation.
argc Specifies the number of command-line arguments
argv is an array of string pointers that provide access to the command-line argument values (these are populated during start-up by a routine that obtains the values from the kernel)

The Structure of a Program

The example below shows the usage of command line arguments:

int
main(int argc, char *argv[])
{
    int i;

    for (i = 0; i < argc; i++)      /* echo all command-line args */
        printf("argv[%d]: %s\n", i, argv[i]);
    exit(0);
}

argc is used as a loop termination condition to print the contents of argv to standard output.

The Structure of a Program

Environment Variables are name-value pairs managed by the operating system that typically store configuration information.
Environment Variables are made available to a C program via a 3rd argument to the main function or via a global variable extern char **environ;.
Most UNIX systems can use:

int main(int argc, char *argv[], char *envp[]);

Environment variables can be accessed in char *envp[].
However ISO C specifies that the main function should only have two arguments. Therefore, your programs should use extern char **environ;

The Structure of a Program

The Structure of extern char **environ;

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The structure of a Program

Upon execution, the machine instruction are loaded from some form of secondary stroage into the main memory.
The start-up routine that initialises the process sets up a contiguous memory space (the image) for the execution.
This memory space consists of:
1. The Text Area - Stores the program instructions (read-only)
2. The Initialised data area - Initialised variables
3. The Uninitialised data area - Empty variables
4. The heap - Dynamically allocated memory (e.g. malloc, calloc)
5. The stack - Automatic variables. (Variables in function calls)

The structure of a Program

Below is an outline of the memory structure

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The structure of a Program

Memory that is dynamically allocated at run-time is sourced from the heap.

#include <stdlib.h>

// size_t is an integer (or long actually) type defined in stdlib
//Allocate a single block of size bytes
void *malloc(size_t size);

//Allocate an noObj lenth array of size bytes
void *calloc (size_t noObj, size_t size);

//Re-allocate the memory at ptr to be of newSize
void *realloc(void *ptr, size_T newSize);

//De-allocate and return the memory to the heap
void free(void *ptr)

Process Control Basics

POSIX defines the process ID a unique (non-zero) integer.
It is used by the operating system to identify a process and it is often used as part of other identifiers.
The scheduler, which is responsible for managing the execution of the process on a unix system, has a process ID of 0.
Like size_t, process IDs are stored in a wrapper type pid_t, which will either be a long or an int. (IS pid_t signed or unsigned? why/why not?)

Process Control Basics

A process can access its own ID using pid_t getpid(void)

#include <sys/types.h>
#include <unistd.h>

pid_t getpid(void);

Process Control Basics

getpid(...) is part of an entire family of system calls:

#include <sys/types.h>
#include <unistd.h>
// Calling Process ID
pid_t getpid(void);

// Parent of calling processes ID
pid_t getppid(void);

// Calling processes user ID
uid_t getuid(void); 

// Calling processes effective user ID
uid_t geteuid(void);

//  Calling processes group ID
gid_t getgid(void);

// Calling processes effective group ID
gig_t getegid(void);

Process Control Basics

Each process has a parent process (exept the scheduler)
A processes parent is the one that created it (getppid(void))
Each process has an owner (i.e. a particular user - getuid(void))
Each owner has specific privileges on the system
Each owner is identified by a unique user ID (uid_t)

Process Control Basics

Every process has an effective user ID. This determines the privileges that a process has for accessing resources such as files.
The effective user ID can change during execution and is determined by a call to uid_t geteuid(void);
The effective user ID of a process is important for various reasons:
1. one is that for one process to send another process a signal,
2. the two processes involved must both have the same effective user ID,or the sender should be root.
3. So you can only kill processes with the same effective user ID.

Process Control Basics

A quick play on turing…

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main(void){
   printf("Process ID: %ld\n", (long)getpid());
   printf("Parent process ID: %ld\n", (long)getppid());
   printf("Owner user ID: %ld\n", (long)getuid());
   printf("Effective user ID: %ld\n", (long)geteuid());
   return 0;
}

Process Control Basics

Each process currently in execution will be in one of five states (on most systmes) that reflect its status.
Remember - Concurrency is the potential for parallelism… if there aren’t enough cores to go around, swapping scheduling is required

State	Menaing
New	Process is being created
Running	The process' instructions are currently being executed
Blocked	The process is waiting for an event (e.g. i/o).
Ready	The process is waiting to run (i.e. can execute but currently scheduled of the processor)
Done	The process is finished and is being de-allocated

Process Control Basics

In UNIX a process can become a zombie!
- A zombie is a child process that has terminated prior to its parent waiting for it’s exit status.
- It remains until it is waited for.
In UNIX a process can also become an orphan:
- An orphan’s original parent has died.
- The orphan is adopted by the Init process!

Process Control Basics

There are a set of possible transitions between the process states:

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Process Control Basics

The act of swapping out a running process(and replacing it with another process) is called a context switch.
The context of a process is the information about the process (and its environment) that the operating system needs to restart it after a context switch.

Process Control Basics

The context contains such things as:
1. the executable program
2. the process state (stack, registers, program counter etc)
3. the memory allocated to the process (static & automatic)
4. the status of the processes i/o
5. the process ID
6. the parent process ID etc
7. scheduling & accounting information
8. signal status

The `fork()` System Call

This is the only way to create a new process (aside from system start-up)

#include <sys/types.h>
 #include <unistd.h>
 pid_t fork ( void );   //Returns -1 on failure

The `fork()` System Call

When executed, fork() creates a new child process.
The function executes once, but returns twice:
1. The return value in the child is 0
2. The return value to the parent is the ID of the child
Both process (parent and child) continue executing from the instruction following the fork() call.
The child is an exact copy of the parent (i.e. the child gets copy of the parents data-space, heap and stack)

The `fork()` System Call

A couple of quick points about fork()
1. Most modern implementations don’t actually create a complete copy of the parent’s memory space.
2. Copy-On-Wite (COW) is used - Memory space between the two process until one of them executes a write operation. A copy in the child is then created

The `fork()` System Call

A call to fork() can fail, resulting in a fork()ing mess if you don’t check for errors
In this situation, fork() only returns once, with a -1
errno is also set:
1. EAGAIN - you already have too many processes going
2. ENOMEM - you haven’t got enough space left for this process
Always check for fork()ing failures!

The Make Utility

make is a UNIX utility for building application from source.
It simplifies complex build processes
Basically the make utility uses the instructions in the makefile to compile the code

The Make Utility

COMPILER = gcc
CFLAGS = -Wall -pedantic

EXES = example01 forkex01  forkex02 forkex03 forkex04 forkex05 forkex06

all: ${EXES}


example01:  example01.c 
    ${COMPILER} ${CFLAGS}  example01.c -o example01
forkex01:  forkex01.c 
    ${COMPILER} ${CFLAGS}  forkex01.c -o forkex01
forkex02:  forkex02.c
    ${COMPILER} ${CFLAGS}  forkex02.c -o forkex02
forkex03:  forkex03.c
    ${COMPILER} ${CFLAGS}  forkex03.c -o forkex03
forkex04:  forkex04.c
    ${COMPILER} ${CFLAGS}  forkex04.c -o forkex04
forkex05:  forkex05.c
    ${COMPILER} ${CFLAGS}  forkex05.c -o forkex05
forkex06:  forkex06.c
    ${COMPILER} ${CFLAGS}  forkex06.c -o forkex06
clean: 
    rm -f *~ *.o ${EXES}

The Make Utility

Constants defined in the first two line
1. Compiler is the GNU C Compiler
2. Compiler options -Wall (warn-all) and -pedantic (looks for non-standard syntax etc.)
3. The compilation targets (EXES) i.e. the Executables we want to compile
Then the instructions for the compilation targets
1. all is the default and is a super-set of the EXES
2. clean executes rm -f *~ *.o ${EXES} to remove the object files and executables for a fresh build
All of your assignments WILL have a makefile submitted with them otherwise they won’t be marked.

Example Applications

We will look at a series of Examples to illustrate the fork() call

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
int main(void){
  pid_t foo;
  foo = fork();
  printf("hey i'm %ld and my foo is %ld\n", 
          (long)getpid(),
          (long)foo);
  return 0;
}

Example Applications

What will this example return?
What is missing?

foo = fork();
if(foo) 
  { /* parent code goes here */ }
else { /* child code goes here */ };

NO ERROR CHECKING!!!

Example Applications

Lets try again:

if((foo = fork()) < 0){ 
    /* fork error */
    perror("error in fork"); 
    exit(1); 
  } else if(foo){ 
    /* parent code here */ 
  } else { 
    /* child code  here */ 
}

/* code here will be executed by both  */
/* child and parent as long as they do */
/* not execute an call to exit or      */
/* abort in their code above           */

This is the template that will get you good marks!

Example Applications

A chain of processes


#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main(void){
  int i, n = 4;
  pid_t childpid;
  for (i = 1; i < n;  ++i)
    if((childpid = fork()) < 0){ 
      /* fork error */
      perror("error in fork"); 
      exit(EXIT_FAILURE); 
    } else if(childpid){ 
      /* parent code */ 
*      break;
    } else { 
      /* child code */ 
    } 
  /* mutual code */ 
  printf("This is process %ld with parent %ld\n",
         (long)getpid(), (long)getppid());
  return 0;
}

Example Applications

*Running this looks like:

turing.Examples> forkex02
This is process 25906 with parent 21711
This is process 25907 with parent 25906
This is process 25909 with parent 25908
turing.Examples> This is process 25908 with parent 25907
Note that the prompt returned before one of the spawned processes.

Example Applications

A fan of processes

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main(void){
  int i, n = 4;
  pid_t childpid;
  for (i = 1; i < n;  ++i)
    if((childpid = fork()) < 0){ 
      /* fork error */
      perror("error in fork"); 
      exit(EXIT_FAILURE); 
    } else if(childpid){ 
      /* parent code */ 
    } else { 
      /* child code  */ 
*      break; 
    }
  /* mutual code */   
  printf("This is process %ld with parent %ld\n",
         (long)getpid(), (long)getppid());
  return 0;
}

Example Applications

Running it looks like:

turing.Examples> forkex03
This is process 25977 with parent 25975
This is process 25976 with parent 25975
This is process 25975 with parent 21711
This is process 25978 with parent 25975
turing.Examples>

We can see who the point of the fan was, just by noticing that 25975 had a different parent than all the rest (and was the parent of all the others).

Example Applications

A tree of processes

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main(void){
  int i, n = 4;
  pid_t childpid;
  for (i = 1; i < n;  i++)
    if((childpid = fork()) < 0){ 
      /* fork error */
      perror("error in fork"); 
      exit(EXIT_FAILURE); 
    } else if(childpid){ 
*      /* parent code */ 
    } else { 
*      /* child code  */ 
    } 
  /* mutual code */
  printf("This is process %ld with parent %ld\n",
         (long)getpid(), (long)getppid());
  return 0;
}

Example Applications

turing.Examples> forkex04
This is process 26140 with parent 21711
This is process 26142 with parent 26140
This is process 26143 with parent 26141
This is process 26141 with parent 26140
turing.Examples> This is process 26146 with parent 1
This is process 26147 with parent 1
This is process 26145 with parent 1
This is process 26144 with parent 1

Whats happening here? We’ve got some poor Orphaned process.
Don’t worry Init (the scheduler with process ID 1)has adopted them!

Example Applications

It’s much better for us to wait for the children?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
int main(void){ 
  int i, n = 4;
  pid_t childpid;
  for (i = 1; i < n;  ++i)
    if((childpid = fork()) < 0){ 
      /* fork error */
      perror("error in fork"); 
      exit(EXIT_FAILURE); 
    } else if(childpid){ 
      /* parent code */ 
      break;
    } else { 
      /* child code  */ 
    }
  /* mutual code */
  printf("This is process %d with parent %d\n",
         getpid(), getppid());
  return 0;
}

Example Applications

Well on turing we see the following:


turing.Examples> forkex05
This is process 27533 with parent 21711
This is process 27535 with parent 27534
This is process 27536 with parent 27535
This is process 27534 with parent 27533
turing.Examples>
They finish and exit in a random fashion.

Example Applications

One final example to demonstrate that memory between processes is Not shared.

int     globvar = 6;        /* external variable in initialized data */
char    buf[] = "a write to stdout\n";

int main(void){
    int var;            /* automatic variable on the stack */
    pid_t   pid;
    var = 88;
    if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1)
        err_sys("write error");
    printf("before fork\n");    /* we don't flush stdout */

    if ((pid = fork()) < 0) {
        err_sys("fork error");
    } else if (pid == 0) {      /* child */
        globvar++;      /* modify variables */
        var++;
    } else {
        sleep(2);       /* parent */
    }
    printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar,var);
    exit(0);
}

Summary

The Structure of a Program
Process Control Basics
The fork() System Call
- Examples

class: middle, center, inverse

Questions?

Reading

Chapter 1, 7 and start chapter 8 from Advanced Programming in the UNIX environment
UNIX Programming Tools - should be mostly revision
Assignment 1 Description
COMP309/509 Study Guide - Pay Special Attention to the Assessment and Study Plan

COMP309/509 - Parallel and Distributed Computing

Lecture 2 - Processes in Unix

By Mitchell Welch

University of New England

Reading

Summary

The Structure of a Program

The Structure of a Program

The Structure of a Program

The Structure of a Program

The structure of a Program

The structure of a Program

The structure of a Program

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

Process Control Basics

The fork() System Call

The fork() System Call

The fork() System Call

The fork() System Call

The Make Utility

The Make Utility

The Make Utility

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Example Applications

Summary

Questions?

Reading

The `fork()` System Call

The `fork()` System Call

The `fork()` System Call

The `fork()` System Call